This invention relates to a magnetic flux detecting circuit which is suitable for measuring a relatively very weak magnetic flux in an environment where a large change occurs in a strong magnetic field. More particularly, this invention relates to a magnetic flux detecting circuit which is applicable to detection of, for example, a nuclear magnetic resonance signal (abbreviated hereinafter as an NMR signal) by a SQUID type flux meter.
A known SQUID type flux meter based on a known principle is provided with a magnetic flux signal transmission path utilizing superconduction as a means for transmitting and detecting magnetic flux to be measured by the flux meter. Such a magnetic flux detecting circuit is disclosed in, for example, JP-A-60-143752.
An NMR apparatus using a SQUID type flux meter is discussed in Applied Physics Letter Vol. 47(6), 1985, pp. 637-639. In this NMR apparatus, an electromagnetic wave is directed toward an object placed in a static magnetic field so as to cause generation of NMR in the object in a manner well known in the art, and the resultant NMR signal generated from the object is received by a high-selectivity LC resonance circuit composed of a receiving coil and a resonant capacitor immersed in a pool of liquid helium and is then detected by the SQUID type flux meter.
In the NMR apparatus using the SQUID type flux meter, a fixed resistance attributable to the loss in the capacitor is inevitably present in the resonance circuit, resulting in generation of thermal noise. Thus, the prior art NMR apparatus has had the problem that the S/N ratio is limited by the thermal noise attributable to the fixed resistance. This problem may be obviated by an arrangement in which, without the use of the resonance circuit, the NMR signal is directly applied to the SQUID type flux meter through a magnetic flux signal transmission path including a superconducting element. However, this arrangement has given rise to degradation of the accuracy of magnetic flux detection, because a shield current corresponding to the strong static magnetic field applied to the NMR apparatus flows in the magnetic flux signal transmission path.
Further, in the case of an NMR imaging apparatus, a gradient magnetic field is superposed on a static magnetic field prior to detection of an NMR signal, and the resultant induced current acts as an additional cause of degradation of the accuracy of magnetic flux detection. Both the strength of the static magnetic field and that of the gradient magnetic field do not change with time during the period of detection of the NMR signal. Therefore, in the following description, the sum of the static magnetic field and the gradient magnetic field will be referred to as a DC magnetic field. The strength of this DC magnetic field does not change with time during the period of detection of the NMR signal but changes in a rectangular fashion before and after that period.